1. Field of the Invention
This invention relates to electrical static eliminators and more particularly relates to corona discharge devices in which discharge electrodes, usually pointed, are coupled to the high side of an A.C. high voltage power supply whose ground side is normally connected to a conductive member or casing adjacently spaced from the discharge points to produce both positive and negative ions in the air gap therebetween. The dual polarity ions emitted by these static eliminators are used to neutralize the surfaces of articles which have become electrically charged by frictional, mechanical, electrical or other generated forces.
The present invention is especially concerned with ionized air blowers in which air, or other gas, is directed over the ionizing points to increase the range of the ionizing field and includes means for balancing the positive and negative ion production so that an equal number of ions of each polarity will be provided thus to insure complete neutralization of the targeted articles.
2. Prior Art
Static eliminators are devices for producing both positive and negative ions in order to neutralize articles or materials which have become charged to a particular polarity or which have a net residual charge in certain zones on the surface. When an A.C. high voltage of fairly high magnitude, for example 15,000 volts, is applied across the discharge points and the grounded casing or shield of such static eliminators, positive and negative ions are emitted from the static discharge electrodes.
While positive and negative ion production may be equal under certain circumstances, in most cases one or the other polarity of ions will predominate depending upon (1) the manner in which the high voltage is connected to the ionizing points, i.e. whether the the points are resistively coupled as in a directly connected bar or capacitively coupled as in a "shockless" bar, (2) the geometry of the static bar, especially the configuration of the grounded portions of the bar and the relationship thereof with respect to the ionizing points, (3) the distance between the static bar and the material to be discharged, and (4) the presence of adjacent grounds with respect to the bar, the latter affecting the amounts of the respective positive or negative ions being emitted from actually reaching the charged material.
In the direct connected bar, such as set forth in U.S. Pat. No. 2,163,294 or U.S. Pat. No. 3,137,806, there is usually a predominance of negative ions produced, even though the discharge points are connected to an A.C. power supply whose positive and negative output voltages are of equal magnitude. The excess negative ion production is as a result of the greater mobility of the negative ions and also because of the inherent characteristics of corona formation wherein ionization occurs over a greater portion of the negative half cycle of voltage in relation to the ionization occurring during the comparable positive half cycle.
In the case of the capacitively coupled bar, such as is described in U.S. Pat. No. 3,120,626, U.S. Pat. No. 3,443,806 or U.S. Pat. No. 3,585,448, there is usually a prevalence of positive ions emitted resulting from the fact that a D.C. voltage is developed across the capacitance in the direction which biases the points slightly positively. The material to be discharged may charge up to the polarity of the predominantly positive charge produced by the capacitively coupled bar or to the preponderantly negative charges emitted by the direct coupled bar.
In U.S. Pat. No. 4,188,530, there is illustrated an extended range static eliminator in which air is blown across the ionizing points through a series of openings toward articles remotely located from the discharge electrodes. The articles themselves are shielded by a casing from corona glow developed about the points.
In U.S. Pat. No. 4,093,543, there is shown and described a balanced ion emission system for a "shockless" capacitively coupled arrangement wherein grounded pointed conductive needles are adjacently and adjustably spaced from some of the pointed discharge electrodes. The points of the "balancing" or control needles are adapted to be adjustably positioned mechanically with respect to the discharge electrodes until an equal number of ions of each polarity are emitted.
In U.S. Pat. No. 4,423,462, a controlled emission static eliminator system is provided by incorporating a biasing circuit in series with the primary of the power supply transformer to control the amplitude and/or duration of the alternating potentials imposed on the corona discharge points. The biasing circuit includes a series-connected diode and a variable resistance in one leg of a parallel network and a capacitor in the other leg. Selecting appropriate time constants for the resistance and capacitance enables the first half of the sine wave to be narrowed while the second half is broadened (or vice versa, depending upon whether the A.C. is directly or capacitively coupled to the points) to yield an equal number of ions of each polarity.
In U.S. Pat. No. 2,879,395, equalization of ion production is accomplished by incorporating a small D.C. power supply either between the bar casing and ground or between the A.C. generator and ground. Adjustment of the magnitude of the auxiliary D.C. voltage provided the desired balance by retarding the output of ions of the opposite polarity.
U.S. Pat. No. 3,714,531 relates to a device for controlling the ratio of positive and negative ions by means of a pair of auxiliary secondary coils, including means for distorting the voltage on the other secondary.
U.S. Pat. No. 4,665,462 concerns an ionizing gas gun for balanced static elimination wherein delay circuitry is included to suspend discontinuance of the positive high voltage for a momentary period subsequent to discontinuance of the negative high voltage.
In U.S. Pat. No. 4,872,083 balance control is achieved by a by-pass resistor across the circuit capacitance to provide a path to ground that bleeds off excess bias so that equal positive and negative ion densities are generated during corona flow.
U.S. Pat. No. 4,417,293 is directed to a static eliminator system employing a pressurized gas which, upon expansion through a nozzle, changes phase and entraps air ions within frozen microparticles, allowing them to be propelled over greater distances. One aspect of this device is said to provide balanced ion emission by embeding the conductive nozzle tip within an insulated jacket while applying the high voltage to the discharge electrodes through a capacitor. The conductive tip or ring electrode providing the ionization field with respect to the discharge electrode is grounded directly.
All of the foregoing balancing systems employ either mechanical or electrical adjustment means to compensate for changes in positive and negative ion flow that are caused by environmental factors and contamination and/or incorporate one or more capacitors in the high voltage circuit before the discharge electrodes or between the ionization field-creating reference electrode and ground to achieve equalization of positive and negative ions.
3. Objectives of the Present Invention
It is therefore an object of this invention to provide a static eliminator system in which ion emission can be balanced without the need for auxiliary mechanical or electrical adjusting devices.
Another object of this invention is to provide a self-balancing ionization circuit for electrical static eliminators wherein equalization of positive and negative ion flow is accomplished without employing capacitors or diodes or other leakage sensitive electrical components intermediate the high voltage power supply and the discharge electrodes or between the field creating reference electrodes and ground.
Still another object of this invention is to provide a self-balancing circuit for static eliminators in which the secondary of the high voltage transformer is totally isolated from ground.
Yet another object of this invention is to provide a highly stable and reliable balancing circuit for extended range static eliminators whose assembly is accomplished with minimal parts and without adjustment mechanisms.
Other objects of this invention are to provide an improved device of the character described which is easily and economically produced, sturdy in construction and both highly efficient and effective in operation.